Volume holograms in photorefractive crystals have generated substantial interest recently for possible applications in high capacity optical storage and optical interconnects. In particular, optical memories based on such volume holograms can potentially combine very large capacities, e.g., greater than 1 Tbyte, with very high data rates, e.g., greater than 1 Gbit/s, and very short access times, e.g., shorter than 1 ms.
A volume hologram can be written in a photorefractive crystal, e.g., iron-doped lithium niobate (Fe:LiNbO.sub.3), by crossing laser beams, e.g., a signal carrying beam and a reference beam, having wavelengths that are absorbed by the crystal. The interference pattern formed by the crossed beams records a corresponding diffraction pattern, i.e., a hologram, in the crystal. A subsequent "read" beam incident on the crystal at a correct angle, e.g., an angle for Bragg diffraction, diffracts from the hologram and reconstructs the signal-carrying beam. A large number of holograms can be recorded in a photorefractive crystal by directing the write and read beams into the crystal at different angles, which is known as angle multiplexing.
In many cases, the write beams initially produce an electronic hologram in which local variation in the density and/or quantum state of electrons or holes in the crystal form the hologram. For example, the optical intensity pattern produced by the write beams can generate free carriers, either electrons or holes, which are trapped at local defect sites to form the hologram. Unfortunately, the electronic grating are volatile and can degrade upon repeated use of reading beams and exposure to moderate temperatures and ambient light. However, thermal fixing of electronic holograms can be used to overcome such volatility.
Thermal fixing involves exchanging electronic holograms with ionic holograms, i.e., holograms formed from ions. Typically, one places the crystal containing the electronic holograms into a high-temperature oven, e.g., greater than 100 degrees Celsius. At such high temperatures, ionic charges become mobile and migrate to the electronic charges that form the electronic hologram thereby compensating the refractive index variations caused by the trapped electronic charges. As a result, there is minimal, if any, diffraction of a read beam at the end of the thermal heating process. After the crystal is cooled, a uniform light beam illuminates the crystal and excites the electronic charges that form the electronic hologram into free carriers. These free carriers migrate uniformly over the crystal volume to reveal a stable ionic hologram. If desired, the ionic hologram can be erased by heating the crystal to even higher temperatures that free the ionic charges that form the ionic hologram, thereby "washing" away the hologram.
Recently, B. Liu et al. (Applied Optics, 37:1342-1349, 1998)described using 10.6 micron radiation from a CO.sub.2 laser to thermally fix a hologram written in a 2 mm thick Fe:LiNbO.sub.3 crystal. The crystal strongly absorbed the 10.6 micron radiation with the absorption constant measured to be between 5000 and 10,000 m.sup.-1. The laser radia ion rapidly and efficiently heated the crystal within its first few hundred microns of thickness, at which point the laser radiation was fully absorbed. Thereafter, thermal conduction within the crystal carried the heat through the thickness of the crystal, thereby thermally fixing the hologram. In some experiments, copper absorbers were introduced into the crystal to more rapidly conduct the heat through the thickness of the crystal.